Reproducible shot hole apparatus

ABSTRACT

A reproducible shot hole for geophysical use wherein a deformable metal liner, such as aluminum, of predetermined diameter, wall thickness, and alloy is selected, and the annulus between the liner and the hole filled with sand-cement under pressure, so that the lined hole will withstand repeated explosions by expanding.

United States Patent [1 1 Wuenschel 1 1 May 28, 1974 [54] REPRODUCIBLESHOT HOLE APPARATUS 2,214,226 9/1940 En glish 166/207 X [751 Inventor:Paul Wuenschel, filenshawv 5215112 511323 ZYL'EZ?.111I1.... 111::1221???? 73 Assignecz Gulf Research & Development 3,175,618 3/l965 Langet al 166/207 X Company, Pittsburgh, Pa.

[ 1 Filed! J 1972 Primary Examiner-David H. Brown [211 App]. No.:267,955

Related US. Application Data [62] Division of Ser. No. 82,907, 0m. 22,1970, Pat. No. ABSTRACT A reproducible shot hole for geophysical usewherein [52] US. Cl. 166/207 a deformable metal liner, such as aluminum,0f prede- [51] Int. Cl E21b 17/02 termined diameter, wall thickness, andalloy is sel Field of Search u lected, and the annulus between the linerand the hole 166/187, 203;285/4L I133R filled with sand-cement underpressure, so that the l V lined hole will withstand repeated explosionsby ex- [56] v References Cited panding.

UNITED STATES PATENTS r 1,834,946 12/1931 Halliburton 166/285 9 Claims,4 Drawing Figures REPRODUCIBLE SHOT HOLE APPARATUS This application is adivision of my prior copending application Ser. No. 82,907 filed Oct.22, 1970, entitled Reproducible Shot Hole, assigned to the same assigneeas the present invention, and now US. Pat. No. 3,693,717.

This invention pertains to geophysical exploration, particularly seismicexploration. in seismic exploration a reproducible source of elasticwaves in the earth, such as is generated by the detonation of a chargeof explosives, is highly desirable. In conventional seismic exploration,a hole is drilled, explosives put in the hole, a relatively large arrayof detectors are placed about the area, and the shot fired. The hole, orthe part of the hole where the shot is detonated, is destroyed in theprocess, so that a shallower level in the same hole must be used, or anew hole must be drilled for the next shot, if a repeat shot is desired.It would be very much preferred to have the ability to fire many chargesfrom one hole at the same depth, with assurance that the waves producedwill be substantially the same from shot to shot, while moving a muchsmaller array of detectors about the area to be explored.

Heretofore, a reproducible shot hole of the character described wascompletely unknown in the art. If the liner or pipe used to protect thehole was made sufficiently strong to resist expansion from theexplosions, then it was found to restrict the radiation of the elasticwaves and hence would be a poor source. Onthe other hand, heretofore, ifthe pipe or liner was made so that it was a good waveradiator, it wasnot strong enough to stand more than one or a very few shots. Theinvention provides steps and apparatus to provide a lined shot holewhich will expand upon repeated firings of charges while maintainingsubstantially constant good wave radiation qualities, and yetmaintaining freedom of passage so that the next charge can be put inplace.

The invention is not to be confused with the many different kinds ofsurface seismic sources which are presently in use. Such surface typeshave many advantages, including claimed reproducibility, but all suffernevertheless from the disadvantage that the waves produced must passfrom the surface device, through the weathered layer before passingthrough the deeper areas of interest. Such surface sources includevehicles which drop heavy weights on the ground, means which vibrate theground, or means which set off small charges of gas in a bomb or otherstrong enclosure which is in contact with the surface. The weatheredlayer is the topmost part of the crust of the earth, varying inthickness according to location, and is highly variable in its abilityto transmit elastic waves. The present invention, like conventionalseismic exploration, utilizes a shot hole drilled to below the weatheredlayer which overcomes this disadvantage in surface types in that thesource, the explosive charge, is fired at a location below the weatheredlayer.

Another advantage of the use of dynamite as a seismic source is theability to excite a specific band of frequencies by the choice of theappropriate charge size and shooting medium. The present invention willapply to a wide range of shooting media consisting of looselyconsolidated gravel at one end of the range to a well cemented shale atthe other end of the range, and it will accommodate linear chargedensities of virtually any size.

Repeat shots from a single hole to a moving relatively small array ofdetectors produces a result virtually parameters to achieve the finalreproducible shot hole.

Examples are set forth in detail in the specification below, butgenerally the method requires balancing the degree of rigidity,consolidation or hardness of the sur rounding formation with thediameter of and ductility of the liner used to case the shot hole. Theinvention includes a liner consisting of certain grades of aluminum, andmeans to protect the junction of the aluminum pipe with the normal steelwell casing used to lower the aluminum liner to the area at which theexplosions will be detonated. The means of supporting a ductile pipe,such as aluminum, along with means to prevent shearing of the pipe atits ends, are important parts, along with others set forth below, of themethod and apparatus of the invention.

The liner can be made of materials other than aluminum. Steel forexample, can be used, but the diameters required when shooting in a softformation would be prohibitive. Aluminum, because of its ductility canbe of reasonable dimensions and this is the reason for the preferencefor liners of the more ductile materials.

The present invention is one of a family of related inventions allpertaining to improvement of the seismic method. The related inventions,all copending with the present invention and allassigned to the sameassignee, in addition to the parent application identified above ofwhich the present application is a division, are:

Clamped Detector by Carl A. Gustavson, Emmett B. Shutes and Paul C.Wuenschel, Ser. No. 255,229, filed May 19, 1972 and now US. Pat. No.3,777,814.

Device for Gripping and lmparting Slack in A Cable by Carl A. Gustavson,Emmett B. Shutes and Paul C. Wuenschel, Ser. No. 256,780, filed May 25,1972.

Precision Seismology by Paul C. Wuenschel, Ser.

No. 227,985, filed Feb. 22, 1972 and now abandoned.

The invention entitled Precision Seismology identified above is anoverall method of using the present invention as well as the otherinventions in a single integrated seismic exploration system.

The above and other advantages of the invention will be pointed out orwill become evident in the following detailed description and claims,and in the accompanying drawing also forming a part of the disclosure inwhich: FlG. l is a cross-sectional view through a portion of the earthshowing a completed but unfired reproducible shot hole of the inventionin place; FIG. 2 is an enlarged view of part of the apparatus of theinvention; FIG. 3 is a composite diagrammatic showing of thereproducible shot hole of the invention after a number of shots havebeen fired and illustrating the configuration of the hole when certainsteps in the method are omitted; and FIG. 4 is an accurate chart whichsummarizes a large amount of experimental 3 work, and which can be usedin practicing the invention."

Referring now to'the drawings and in particular to FIG. '1, there isshown a cross-section of the earth in which the shots are to be fired.The method of the invention will be set, forth below in conjunction withthe description of HG. 1, it being understood that FIG. 1 showsconditions after the hole has been prepared and is ready to have thefirst shot fired.

The first step is to determine the nature of formation 10, specificallythe rigidity of the rock. This can be determined from previousexperience or by drilling a small diameter hole, studying the samplesobtained while drilling, observing the drilling rate, and running logsin the hole after drilling. With this information and with knowledge ofthe seismic frequency band desired, one skilled in the art will know thelinear charge density of the explosive required. The linear chargedensity and the rigidity of the rock together in turn determine theselection of the material for and diameter of the liner 14, which, ofcourse, in turn determines the final hole diameter 12. It is desired tohave the liner as snugly fitted in the hole 12 as possible, so-thattheliner will be supported by the formation.

The general considerations in selecting the liner 14 include that itsductility should be inversely proportional to the degree ofconsolidation. Restated, the softer the formation, the more ductile thematerial of the liner. The reason for this relationship is that theliner 14 will expand, more against a softer formation than it willagainst a harder formation. The invention accommodates to this fact byproviding the ability to expand rather than by trying to make the linerso strong that it cannot expand. Heretofore, and in preliminary testsrun in conjunction with development of the present invention, it wasfound that'less ductile materials, such as various grades of steel,promptly burst after one or two explosions because they did not have theability to expand in response to the explosive force. As is evident,once the liner is burst, the hole quickly fills with debris which entersthe hole via the fissure, thus precluding further access to that depth.

Wall thickness of the liner is not crucial when the pipe is supported bya formation, but should be in ratio to the pipe diameter at about 3:100.If the wall is too thin, the pipe will break due to small irregularitiesin the supporting medium. if it is too thick, the stiffness will limitthe radiation efficiency of the explosion. The tolerance on the valuegiven above is believed quite large but has not as yet been establishedby experiment.

spect to the borehole. Conventional well cement'has been found to be notsatisfactory in that it does not have sufficient strength. Accordingly,the invention comprises the use of a sand-cement mixture filling theannulus between the liner l4 and casing 16 and the borehole .12. For thesame reason, i.e., stabilizing the borehole, it has been advantageous tosqueeze" the sandcement 20 into the annulus. The reason for pressurizingthe sand-cement during its placement is to assure that any voids, orsmall local unconsolidated regions, are filled to provide a uniformstrength around liner 14. Such local discontinuities, are caused by, forexampie, gravel streaks in formation 10, sand and the like. Such a weakplace is indicated in FIG. 1 by reference numeral 22, and is shown afterthe squeezing of sandcement 20 as being filled with mixture 20. Aftersetting, the strength of the support around liner 14 will be uniform.

The finai step is locating the charge assembly 24 in the completedreproducible shot hole. Charge assembly 24 comprises, starting at'thelower end, a weight 26 to pull the remainder of the assembly 24 down thehole with the aid of gravity. A cable or other suitable means 28interconnects weight 26 with the charge 30. The charge 30 may be anyexplosive normally used in seismic exploration, one such explosive beinga type of dynamite sold under the DuPont registered trademark Nitramon"having a strength of 2 pounds per foot. The legends on FIG. 4 indicatethe tradenames and strengths of other charges which may be used. The topof charge 30 is connected by a cable 32 to a centralizing sub-assembly34 comprising upper and lower bow springs 36 interconnected by a supportmember 38 The cable 32 bridges across the sub-assembly 34 to deliverelectricity to the charge. The supporting member 38 is preferably madeof wood or other light material since it is desired that it remainintact as it is frequently shot out of the hole upon explosion of charge30. lt is desired that sub-assembly 34 remain intact so that it can beremoved and reused and not hinder placement of the next shot. Further, ahard wood such as maple is preferred to a soft wood such as pine becausethe latter tends to shatter and the hard wood does not. The use ofcentralizer sub-assembly 34, or other means to centralize the charge 30in the hole, is important to successful use of the invention in that ifthe charge be very much closer to one side of the hole than to theother, that side is more susceptible to bursting. As is conventional,the entire hole may be filled withwater to couple the explosive force tothe borehole. Water is readily available and inexpensive, but otherfluids such as drilling mud could be used in other circumstances.

Referring now to FlG. 2, the joint protection means 18 are shown indetail. The essence of the manner of protection of the joint between thesoft aluminum liner l4 and the hard steel casing 16 is the provision ofa tapered sleeve 40 extending from the joint in closely spaced relationto each end of the liner 14 to provide a differential and increasingforce from the end coupling 42 at each end of the liner 14 is modifiedfor cooperation with the liner by cutting away part of one set ofthreads as at 44 and by undercutting the outside of the couplingopposite cut-out 44 as at 46. A taper 47 is provided at the end ofundercut 44. Thus, an annular flange 48 is formed between cut-out 44 andundercut 46. A shoulder 50 is formed at the inner end of cut-out 44against which an end of the liner l4seats. The thick end of sleeve 40fits in undercut 46 on the outside of flange 48 and is fixed thereto bysuitable means such as a bead of welding 52. The assembly of sleeve 40,flange 48, and liner 14 is finally secured together by a row of bolts 54provided in mating openings in these three members and threaded into theliner 14. The heads of the bolts are preferably of the socket type sothat a minimum obstruction is provided on the outside of the sleeve 40.The inside surface 56 of the sleeve is cylin drical, and the outsidesurface 58 thereof tapers towards surface 56 to provide a thinnestportion of the sleeve at the end thereof. furthest from bolts 54. Aclearance space, of roughly the thickness of flange 48, is providedbetween sleeve 40 and liner 14.

Thus, the joint between the casing string 16 and the liner 14 isprotected by virtue of the tapered sleeve 40 in that the liner will havevery little additional resistance to expansion opposite the thin end ofthe sleeve, but that such resistance will gradually increasemovingtowards the bolts 54. As will appear more clearly below, absent thesleeve, the liner would simply shear off the coupling on the lower endof casing 16.

During the development of the present invention a relatively largenumber of tests were run. Initially, these tests were in the nature offeasibility studiesto see if a reproducible shot hole could actually bemade and to verify that the deformation produced by several smallcharges is equivalent to the deformation produced by one large chargewhose magnitude is the sum of the small charges. This was found to betrue for unsupported pipes by Johnson, ct al., and it was necessary toestablish the same relationship for supported pipes. This work isreported in a paper entitled The Explosive Expansion of UnrestrainedTubes by W. Johnson, E. Doege, and F. W. Travis, in the 1964-5Proceedings of the Institution of Mechanical Engineers, Vol. 179, Part1, No. 7, pages 240-256.

After this initial work was successfully completed it was then necessaryto evaluate the support provided by various types of wall rock. Twoextreme cases as to rigidity were chosen, a loosely consolidated graveland a well silicified shale whose compressional velocity is 12,000 feetper second. An aluminum pipe of alloy 3003- with a wall thickness todiameter ratio of 0.03 was found to withstand a strain of 180 percentbefore breaking when expanded with gravel support and was emplacedaccording to the invention. With this knowledge and data obtained fromall experiments conducted in gravel and hard rock it became possible tosummarize the design parameters for a viable tool in a single graph, asis reproduced in FIG. 4.

The ordinate in FIG. 4 is the pipe diameter required to permit thedesired number of repeat shots at various linear charge densities asgiven by the abscissa and identified by DuPont tradenames andcorresponding weights for a total strain of 180 percent. The parametersused in FIG. 4 are the supporting medium. Loosely consolidated gravelprovides the least support while hard rock provides the most support. Anintermediate medium of moderately packed gravel lies in between thesetwo curves. Other media such as clay, shale, porous limestone, etc. willyield curves falling between the moderately packed gravel and the hardrock.

' The curves of FIG. 4 are applicable for a strain of 180 percent andpertain to ductile materials like aluminum.

Similar curves can be made for other strains and thereby be applicableto other materials.

Several general principles can be drawn from the specific data presentedin FIG. 4, as well as from other tests not included in drawing thecurves. Two of these principlesare illustrated in the showing of FIG. 3.After each shot or each few shots the borehole diameter was physicallymeasured with the use of hole calipering tools. The data presented indiagrammatic form in FIG. 3 was taken from these caliper logs. The solidline labeled 60 shows the measured condition of the borehole when theinvention was used in a preferred form. The dotted line 62 was run withall conditions the same as in the test indicated by the line 60 with theexception that the sandcement was not put in under pressure. Note thatthe liner 14 was pushed deep into the loosely consolidated portions ofthe formation. The liner will burst when the strain exceeds a criticalvalue which depends on the metal used, for aluminum this value is aboutpercent. The final dot-dash line 64 shows the effect of the protectionmeans 18 and the tapered sleeve 40. Absent sleeve 40, the liner promptlyshears at its joints with the casing 16. Note the gradual and gentleexpansion of the liner over the outer half of each sleeve. As fabricatedin the successfully used and tested embodiments of the invention, thesleeve 40 had a length of about 3 feet tapered over only its outer half.The liner 14 had a length of about 20 feet in all tests.

While the invention has been described in detail above, it is to beunderstood that this detailed description is by way of example only, andthe protection granted is to be limited only within the spirit of theinvention and the scope'of the following claims.

I claim:

1. In apparatus for making a reproducible shot hole in the earth, thecombination comprising a string of casing which includes a length ofshot hole liner to be positioned at the depth at which shots are to befired; a coupling having means to mount an end of said casing string andan end of said liner therein, a tapered sleeve surrounding the endportion of said liner and having its thickest end at said coupling; andmeans to join said thick end of said sleeve, the end of said liner, andsaid coupling together; whereby said liner at said coupling is highlyresistant to expansion and decreasingly less resistant to expansion atpoints on said liner spaced along said sleeve moving away from saidcoupling.

2. The combination of claim 1, said liner consisting of aluminum.

3. In apparatus for a reproducible shot hole the improvement comprisinga string of casing cemented in the shot hole to a depth below theweathered zone at which the shot is to be fired, said string of casinghaving a cylindrical liner of ductile material through the interval ofthe hole at which the shot is to be fired, a steel sleeve at each end ofthe liner surrounding and spaced slightly from the liner and extendingfrom the end of the liner toward the center thereof for a distance tosupport the ends of the liner and leave the liner unsupported in theinterval at which the shot is to be fired.

4. The apparatus of claim 3 in which each of the sleeves is of greatestthickness at the end of the liner and tapers to a reduced thickness atthe end of the sleeve nearest the midpoint of the liner.

5. The apparatus of claim 3 in which the liner is in the borehole withina sheath of a cement-sand mixture set under an elevated pressure to fillvoids and reinforce weak formations surrounding the liner.

6. Apparatus as set forth in claim 3 in which the liner is aluminum.

7. Apparatus as set forth in claim 3 in which the ratio of the thicknessof the liner to its diameter is approximately 3:100.

8. Apparatus for a reproducible shot hole comprising an upper section ofsteel casing and a lower section of steel casing, an upper coupling atthe lower end of the upper section and a lower coupling at the upper endof the lower section, an aluminum liner extending between the uppersection and lower section of casing and having its upper end mounted inthe upper coupling and its lower end mounted in the lower coupling, anupper sleeve surrounding and secured to the upper coupling and extendingtherefrom downwardly part of the distance to the lower end of the liner,a lower sleeve surrounding and secured to the lower coupling andextending upwardly therefrom a part of the distance to the lower end ofthe upper sleeve to leave a central portion of the liner unsurrounded bythe sleeve.

9. Apparatus as set forth in claim 8 in which the sleeves taper from asection of maximum thickness at the ends secured to the couplings.

1. In apparatus for making a reproducible shot hole in the earth, thecombination comprising a string of casing which includes a length ofshot hole liner to be positioned at the depth at which shots are to befired; a coupling having means to mount an end of said casing string andan end of said liner therein, a tapered sleeve surrounding the endportion of said liner and having its thickest end at said coupling; andmeans to join said thick end of said sleeve, the end of said liner, andsaid coupling together; whereby said liner at said coupling is highlyresistant to expansion and decreasingly less resistant to expansion atpoints on said liner spaced along said sleeve moving away from saidcoupling.
 2. The combination of claim 1, said liner consisting ofaluminum.
 3. In apparatus for a reproducible shot hole the improvementcomprising a string of casing cemented in the shot hole to a depth belowthe weathered zone at which the shot is to be fired, said string ofcasing having a cylindrical liner of ductile material through theinterval of the hole at which the shot is to be fired, a steel sleeve ateach end of the liner surrounding and spaced slightly from the liner andextending from the end of the liner toward the center thereof for adistance to support the ends of the liner and leave the linerunsupported in the interval at which the shot is to be fired.
 4. Theapparatus of claim 3 in which each of the sleeves is of greatestthickness at the end of the liner and tapers to a reduced thickness atthe end of the sleeve nearest the midpoint of the liner.
 5. Theapparatus of claim 3 in which the liner is in the borehole within asheath of a cement-sand mixture set under an elevated pressure to fillvoids and reinforce weak formations surrounding the liner.
 6. Apparatusas set forth in claim 3 in which the liner is aluminum.
 7. Apparatus asset forth in claim 3 in which the ratio of the thickness of the liner toits diameter is approximately 3:100.
 8. Apparatus for a reproducibleshot hole comprising an upper section of steel casing and a lowersection of steel casing, an upper coupling at the lower end of the uppersection and a lower coupling at the upper end of the lower section, analuminum liner extending between the upper section and lower section ofcasing and having its upper end mounted in the upper coupling and itslower end mounted in the lower coupling, an upper sleeve surrounding andsecured to the upper coupling and extending therefrom downwardly part ofthe distance to the lower end of the liner, a lower sleeve surroundingand secured to the lower coupling and extending upwardly therefrom apart of the distance to the lower end of the upper sleeve to leave acentral portion of the liner unsurrounded by the sleeve.
 9. Apparatus asset forth in claim 8 in which the sleeves taper from a section ofmaximum thickness at the ends secured to the couplings.